Adam Clarke on Matthew 13:21

Yet hath he not root in himself - His soul is not deeply convinced of its guilt and depravity; the fallow ground is not properly ploughed up, nor the rock broken. When persecution, etc., ariseth, which he did not expect, he is soon stumbled - seeks some pretext to abandon both the doctrine and followers of Christ. Having not felt his own sore, and the plague of his heart, he has not properly discovered that this salvation is the only remedy for his soul: thus he has no motive in his heart strong enough to counteract the outward scandal of the cross; so he endureth only for the time in which there is no difficulty to encounter, no cross to bear.